Note: Descriptions are shown in the official language in which they were submitted.
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DIFFUSER FOR CENTRIFUGAL COMPRESSOR
RELATED APPLICATION
This application claims benefit of the 24 September 2003 filing. date of
United States
Provisional Application Number 60/505,885.
FIELD OF THE INVENTION
This invention relates generally to the field of turbo machines and more
particularly to
a diffuser for a centrifugal compressor.
BACKGROUND OF THE 1NVENTION
Centrifugal compressors are known to utilize diffusers for converting a
portion of the
kinetic energy of a working fluid leaving a compressor wheel into static
pressure by
slowing the flow velocity of the working fluid through an expanding flow
volume
region. Diffusers may incorporate airfoils, commonly called vanes, for
directing the
working fluid through the expanding volume to enhance this process, with each
vane
having a pressure side and a suction side relative to an angle of attack of
the incoming
working fluid. FIG. 1 illustrates how a prior art diffuser 10 may develop a
large flow
separation zone 12 on the suction side 14 of a diffuser vane 16 under certain
conditions. The flow separation zone 12 is essentially a flow boundary layer
that has
a lower velocity than the remainder of the flow and therefore hinders the
overall fluid
flow rate. The flow separation zone 12 creates a distorted exit flow 18 from
the
compressor, reducing the efficiency of the compressor and potentially leading
to surge
and stall of the compressor, with resultant damage to the compressor and/or a
downstream turbocharged engine. For the embodiment of a compressor used as a
turbocharger for the diesel engine of a railroad locomotive, the compressor is
most
vulnerable to such surge and stall events when the locomotive is operating at
high
altitude, low ambient temperature, and high manifold air temperature; for
example
when just exiting a high altitude tunnel.
As illustrated in Fig. l, the conventional wisdom for the design of compressor
diffuser
vanes 16 is to provide uninterrupted surfaces 20 from the leading edge 22 to
the
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trailing edge 24 of the vanes to maximize the surface area of the vane exposed
to the
differential pressure between the suction side 14 and the pressure side 26.
The
position and angle of the vane is chosen as a compromise between avoiding
stalling of
the flow and maintaining efficient pressure recovery for the angles of attack
of the
various incoming air flow streams that were anticipated to impinge upon the
vane.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of flow boundary separation from the suction side of
a
diffuser vane in a prior art centrifugal compressor.
FIG. 2 is an illustration of the flow conditions on the suction side of a
slotted diffuser
vane.
FIG. 3 is a compressor map for a prior art cascade diffuser.
FIG. 4 is a compressor map for a cascade diffuser having slotted vanes.
FIG. 5 is a partial cross-sectional view of a compressor having slotted
diffuser vanes.
FIG. 6 illustrates the throat region of a slotted vane island diffuser.
FIG. 7 illustrates the throat region of a slotted vane cascade diffuser.
FIG. 8 is a partial cross-sectional view of a compressor diffuser having a
plurality of
flow passages from the pressure side to the suction side of a vane.
FIG. 9 is a perspective view of a portion of a diffuser having slotted vanes
with
leading edge support members.
DETAILED DESCRIPTION OF THE INVENTION
Through experimentation, the applicants have found that in the prior art
centrifugal
compressor designs for maximizing diffuser performance, efficiency can be
reduced
and the vane made more likely to stall, leading to compressor surge due to the
formation of a flow separation zone at the suction side of the vane.
Furthermore, and
as explained in detail hereinafter, the applicants have found that by forming
a flow
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opening allowing a portion of the working fluid to flow through or over the
vane from
the pressure side to the suction side of the vane, the flow separation zone
can be
reduced or eliminated, efficiency increased, and the likelihood of stall or
surge
reduced.
An improved diffuser 30 for a centrifugal compressor is illustrated in FIG. 2.
The
diffuser vanes 32 each include an opening allowing a portion 38 of the working
fluid
to pass from the pressure side 40 to the suction side 42 of the airfoil. The
opening is
illustrated in FIG. 2 as slot 34 formed between a leading edge portion 36 of
the vane
32 and the mating diffuser wall member. The mating wall member is not
illustrated in
FIGS. 1 and 2 so that the airfoils and working fluid flow paths may be more
clearly
seen; however, one will appreciate that opposed wall members of the diffuser
are
positioned above and below and extending between the vanes to define a flow
path for
the working fluid there between. The slot 34 allows a portion 38 of the
working fluid
to pass over the vane 32 from the pressure side 40 to the suction side 42,
thereby re-
energizing the flow boundary region 43 of the working fluid flowing against
the
suction side 42, and thereby minimizing any flow separation zone 44 that may
tend to
form. It is believed that the portion 38 of the working fluid passing over the
vane 32
creates a vortex that interferes with the growth of the flow separation zone.
A
comparison of FIG. 1 and F1G. 2 schematically illustrates the reduced size of
flow
separation zone 44 and the improved uniformity of exit flow 46 of vane 32
compared
to prior art vane 16 under the same inlet angle of attack and flow conditions.
A comparison of FIGs. 3 and 4 provides a graphical illustration of the
improved
compressor performance that may be achieved with the slotted diffuser vane 32
of
FIG. 2. FIGs. 3 and 4 are traditional compressor maps, and each figure
includes a
plurality of generally horizontal lines that represent the compressor's
performance
(temperature corrected flow rate verses compressor stage pressure ratio) at a
respective temperature corrected compressor operating speed. FIG. 3 is a
performance map 50 for a compressor utilizing a prior art cascade diffuser
having
vanes of the type shown in FIG. 1. FIG. 4 is an equivalent map 52 for the same
compressor having been modified to include flow slots 34 similar to those
illustrated
in FIG. 2. Notice the extended range of flow rates that are available at any
given
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compressor operating speed (i.e. the longer horizontal portion of the curves
extending
to both relatively lower and higher flow rates) for the compressor of FIG. 4.
Surge
lines 54, 56 are constructed by connecting the left end (low flow) points of
the various
corrected speed lines. In general, under the same conditions, the compressor
of FIG.
4 can be operated to a lower flow rate before a stall event will occur. Also
notice that
the right sides of the various performance lines of the improved design of
FIG. 4
generally do not drop downward as quickly as those of the performance lines of
FIG.
3. Lines 58, 60 (choke flow) are constructed by connecting the right end (high
flow)
points of the various corrected speed lines. This difference is an indication
of an
improved high flow rate efficiency of the compressor of FIG. 4 when compared
to the
compressor of prior art FIG. 3. The improved performance resulting from the
use of
flow openings 34 may provide improved margin against surge/stall events, or it
may
be utilized by the component designer in other ways to improve the overall
performance of the component design.
Flow opening slots 34 are gaps formed between the respective vane 32 and the
mating
diffuser wall (not shown in FIG. 2) when the diffuser 30 is assembled. The
vanes 32
are typically formed to be integral with a base plate, such as by machining
these
components from a single piece of material or by welding separately formed
vanes to
a base plate. A notch or groove may be machined into a top surface of each
vane to
extend between the pressure side 40 and the suction side 42 prior to that
surface being
connected to a respective mating wall. The notches represent material removed
to
define the flow slots 34 along the leading edge portion 36 of the vanes
proximate the
mating diffuser wall when the diffuser 30 is assembled.
FIG. 5 is a partial cross-sectional view of a compressor 60 including the
improved
diffuser 30 of FIG. 2. Impeller 62 is rotatable between an air inlet housing
64 and a
compressor casing 70 to provide a flow of compressed working fluid 67 through
diffuser 30 and into the blower casing 66. The diffuser vane 32 is situated
between
opposed diffuser walls; in this embodiment one wall being the diffuser base
plate 68
and the other opposed wall being the compressor casing 70. Flow slot 34 is
formed in
the leading edge of the diffuser vane 32 adjacent the casing 70.
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FIG. 6 is a partial top sectional view of an improved vane island diffuser
(wedge
diffuser) 72. Fig. 7 is a partial top sectional view of an improved cascade
diffuser 74.
The throat 76, 78 of these respective diffusers 72, 74 is the distance between
adjacent
vanes at their closest points along their respective chord lengths. The flow
openings
of the present invention may extend from the vane leading edge or from a point
downstream of the leading edge along a suitable distance along the chord
length of the
vane, for example in the range of from at least 5% to no more than 25% or no
more
than 38% of the chord length of the vane in various embodiments. A flow slot
may
extend along only a leading edge portion of the vane upstream from a throat of
the
vane and not from the throat to points downstream of the throat, as
illustrated in FIG.
6.
The depth of the slots may be of a suitable dimension, such as no more than
10% of
the height of the vane perpendicular to the vane chord in one embodiment, or
no more
than 5% of that height in another embodiment. Because the opening defines a
fluid
flow path, there may be a practical minimum established in order to avoid
plugging
due to debris carried by the working fluid, for example no less than 50 mils.
The precise location and geometry of the flow opening from the pressure side
to the
suction side of a diffuser airfoil may vary for different applications. The
flow path
may be a single opening or a plurality of openings spaced apart along the
chord of the
vane. Each of such multiple openings may have the same or different
geometries. It
is believed that the flow slots are best formed at the juncture of the vane
and one of
the respective opposed walls, since it is along this corner that flow
separation
generally first develops. However, the opening may be formed in the vane
somewhat
removed from the adjoining wall in certain embodiments or it may be formed in
the
mating wall member, as illustrated in FIG. 8. FIG. 8 is a partial cross-
sectional view
of view of a compressor diffuser 80 having a vane 82 connected between opposed
walls 84, 86 for directing a flow of a working fluid 88. At least one hole 90
is drilled
through the vane 82 to have an inlet on the pressure side and an outlet on the
suction
side proximate a first of the walls 84 to allow a first portion of the working
fluid 88 to
flow there through. The outlet of the hole 90 is located on the suction side
of the vane
82 upstream from a throat location 89 (illustrated by dashed line). A second
portion
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of the working fluid 88 may be permitted to flow from the pressure side to the
suction
side through an opening formed as a groove 92 in the second of the walls 86.
The
location of the holes 90 and groove 92 along the chord of the vane 82 may be
selected
to optimize the impact of the respective bypass flows on the formation of a
downstream flow separation zone. From a manufacturing perspective, it may be
convenient to form a flow opening as a machined notch between the pressure and
suction side surfaces along a top surface of a vane, and/or as a machined
groove into a
diffuser wall, prior to the wall being mated to the vane. In certain
embodiments it
may be desired to form a flow slot on both opposed sides of the vane proximate
both
opposed diffuser walls.
In general, it may be desired to create the minimum amount of bypass flow over
the
vane that is necessary to suppress expansion of the flow separation zone on
the
suction side of the vane to the extent necessary to achieve a desired degree
of
improvement in the exit flow distribution and in the low and high flow
performance
of the diffuser. Generally, more bypass flow will result in a greater
improvement in
low and high flow performance with a corresponding decrease in peak efficiency
of
the compressor, thus suggesting a cost/benefit analysis for arriving at
optimal bypass
flow opening geometry for a particular application. For a turbo-charger
compressor
such as used in modern locomotives manufactured by the assignee of the present
invention, a typical diffuser vane may have a chord length of about 4 inches
and a
vane height of about 0.9 inch. Flow slots having widths of 0.050 inches and
0.085
inches and extending along about 15% of the chord length have been tested with
success in such units.
FIG. 9 is a partial perspective illustration of a further embodiment wherein a
support
connection 94 is used between the leading edge 96 of the vane 98 and the
diffuser
wall 100 in order to provide mechanical support for the leading edge 96 of the
vane
98, if necessary or desired. The flow opening 102 extends along the leading
edge
portion of the vane 98 downstream from the support connection 94. The support
connection 94 may be an integral extension of the vane material or it may be
fabricated such as by welding or it may be a separately attached piece of
material. In
one embodiment, the flow opening 102 may begin about 0.1 inches back from the
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leading edge 96 for a vane 98 such as described above for a locomotive turbo-
charger
compressor. The leading edge support may be applied to address diffuser, vane
vibration, particularly on thin-vaned diffusers. Such vibration may be excited
by
compressor wheel blade and diffuser vane flow interaction. The support 94
creates a
mechanical constraint for the leading edge 96 of the vane 98, and therefore,
it
prevents excessive vibration that may be detrimental to the life of the
component.
While various embodiments of the present invention have been shown and
described
herein, it will be obvious that such embodiments are provided by way of
example
only. Numerous variations, changes and substitutions will occur to those of
skill in
the art without departing from the invention herein. Accordingly, it is
intended that
the invention be limited only by the spirit and scope of the appended claims.
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